Coupling system



Feb. 16, 1932. w, A. MacDONALD 3,845,306

COUPLING SYSTEM Original Filed Feb. 15. 1929 2 Sheets-Shea?I 2 ATTORNEYS Patented Feb. 16, 1932 UNITED STATES PATENT OFFICE WILLIAM A. MACDONALD, F LITTLE NECK, NEW YORK, ASSIGNOR T0 HAZELTINE COB- IPORATION, A CORPORATION OF DELAWARE COUPLING SYSTEM Original application led February 15, 1929, Serial No. 340,244, and in Canada January 2, 1930. Divided and this application filed November 25, 1930. Serial No. 498,026.

The present application is a division of application Serial No. 840,244, filed February This invention relates to radio coupling systems, particularly to transformers for use in coupling two successive Vacuum tubes in a radio-frequency amplifier, and also in coupling an antenna system to such an amplifier,

intended for the efficient reception of two or more diderent frequency, or wavelength bands.

To this end the invention employs coupling transformers having a multiplicity of windings so related to each other that the several primary coils operate as a single primary and the several secondary coils operate as a single secondary over one given frequency band, but that only certain of the primary and secondary coils operate as a single trans- 20 former over another given frequency band.

A. further feature of the invention is an improved frequency-ampliication characteristic by reason of the provision of precalculated reactions' between the various coils.

In the past, three expedients have been most commonly employed for the reception of two or more Widely differing frequency bands. The first is the use of as many separate receivers or ampliliers as there are different bands to be received. The second consists in employing a. separate transformer especially designed for each frequency band; thel various transformers having plug terminals which, when inserted in a suitable receptacle, automatically establish the required electrical connections with the associated apparatus. The third expedient consists in permanently mounting as many physically separate trans- 40 ormers as there are different frequency bands to be received, and shifting the connections of the transformers with the associated apparatus from. .one transformer to another by means of an appropriate multi point switching device.

The main objection to the iirst expedient is obviously that it is economically impracticable because of the attendant duplication of apparatus. rIhe second and third expedientsl have, therefore, been proposed to obviate the main objection to the first.

rl`he second-named arrangement is electrically sound, but is clumsy and undesirable from the point of view of operation, because a shift from one frequency band to another necessitates the opening of the receiver cabinet, the removal of one coil and the ,insertion of another. This operation is especially undesirable when the arrangement is employed in a' multistage amplifier because in such event it is necessary to change a number of transformers, which operation entails a considerable delay in tuning. Furthermore, in this second arrangement, it frequently happens that the spare transformers,

1. e., those not for the moment employed Within the receiver, become damaged or lost. Finally, the construction and duplication of such fplug-in transformers is necessarily expensive.

In the case of the third-mentioned expedient, the use of two or more entire Vsets of transformers within the receiver is objectionable because of the additional space required and because of the increased cost. Furthermore, all the switching devices for this apparatus thus far proposed are complicated, expensive, and subject to failure both electrically and mechanically. In the case of multi-stage amplifiers wherein large degrees of amplification are required, these complicated switching devices introduce both resistance and capacity losses which are detrimental to satisfactory operation of the amplifier; and in'addition, the exposed highpotential portions of the switch and its associated connections introduce detrimental capacitive couplings which are diiicult, if not impossible, to eliminate and which, in

many cases, result in uncontrollable regeneration or oscillations.

The ,present invention contemplates not only the design of transformers of high efficiency whereby high amplification may be attained, but utilizes a more simple electrical and mechanical structure than has heretofore been employed in the reception of a plurality of frequency bands. An additional advantage of the invention results from the design of the transformers such that uniform, or substantially uniform, amplification is attained over one or more of the frequency bands, as desired. rl`his invention overcomes all of the disadvantage of the prior arrangements, as pointed out, by providing a single transformer structure which may be permanently fitted in the receiver or amplifier', but which includes at least as many sets of windings as there are frequency bands to be received; the change from one frequency band to another being accomplished by short-circuiting or open-circuiting one or more of the unused coils in each transform-er, such as the secondary coil for example, in which case the primary coils are always connected in circuit. Y

Before describing the invention with the aid of the accompanying drawings, it should be pointed out that this invention is not limited to use in tuned amplifier circuits but may be used with good success in untuned amplifiers. The invention is adapted to be used not only in colnbination with the common three-element tube, but also with the four-element tube, commonly known as the shieldor screen-grid tube (the particular type of tube employed having no special relation to this invention); it may likewise be employed as a coupling device in amplifiers whether or not they include neutralization, and for coupling the amplifier to an antenna as well as for coupling one vacuum tube with another. By neutralization is meant the neutralization of the capacitive coupling between two elements or electrodes of a vacuum tube together with the associated wiring, whereby the tendency towards the production of oscillations is reduced or eliminated.

Although Figure l of the drawings shows a typical physical embodiment of a coupling transformer in accordance with this invention, the description of the figures is so arranged that otheriigures relating to the application of the coupling transformer v4are first described in detail. This knowledge of the special applications of the coupling transformer facilitates understanding the significance of its structure.

'Referring to the drawings:

Fig. 1 is a cross-sectional view of a coupling transformer arranged in accordance with this invention.

Fig. 1a and Fig. 1b show the amplification frequency curves of the transformer of Fig. 1 over two different frequency bands.

Fig. 2 shows a simple neutralized radiofrequency amplifier circuit including an intertube coupling system arranged in accordance with this invention. A third circuit is provided for neutralizing the effect .of the grid-plate capacity of the tube.

Fig. 3 shows a radio-frequenc amplifier, similar to that of Fig. 2 coupled y a special system, corresponding to the intertube coupling system hereinbefore described, to an antenna circuit.

Fig. 4 illustrates the approximate fre- 'quency characteristics of the antenna system of Fig. 3 over the particular two frequency bands considered.

The circuit of Fig. 5 is a modification of Figure 2 by which the amplification/frequency curve is made more nearly level.

Fig. 5a illustrates approximately the amplication/frequency curve obtained from the arrangement of Fig. 5.

The circuit diagram of Fig. 2 represents a tunedv radio-frequency amplifier including two vacuum tubes-A1 and A2 coupled by means of a system in accordance with this invention, including a coupling transformer comprising two primary windings L1 and L2, two secondary windings L3 and Lgand two neutralizing windings N1, and N2. In effect, these six windings constitute ltwo transformers, the first transformer L1, N1 and L3 being wound to comparatively small inductance to cover a high-frequency band; and the second transformer L2, N2 and L4 being wound to comparatively high inductance to cover a low-frequency band, altho, as is hereinafter pointed out, the two transformers may be designed to react upon each other in a definite, precalculated manner. For the convenience of the present discussion, the high-frequency tuning band may be considered as that between 500 and 1500 kilocycles, and the low-frequency tuning band as that between and 300 kilocycles, these two bands being those at present employed for radio broadcasting in England. This particular example, however, is given only as a specific illustration of one embodiment ofthe invention; for obviously, other frequency bands may as well be accommodated. For instance, the invention may be used to advantage in radio receivers adapted to respond to the two broadcasting bands at present in effect in the United States, i. e., the band between 500 and 1500 kilocycles and thatbetween 5000 and 7500 kilocycles. Y

When used for the reception of the lower radio-frequency band, the switch Sl is opened, whereby the primary inductances L1 and L2 are in series in the plate circuit of vacuum tube A1, and the secondary windings Lz.1 and L4 are in series in the input circuit of lll l vacuum tube A2. These two sets of windings then function like a. single low radiofrequency transformer. Variable condenser C, connected to the extreme terminals of L2 and L4, then operates to tune the coupling system over this band. When it is desired to receive over the high frequency band; the switch S1 is closed, thus short-circuiting the low-frequency secondary L2, but leaving in circuit the high-frequency secondary L2 with the variable tuning condenser C connected across its extremities. As is represented in the diagram, switch S1 may be a simple single arm, single-contact switch, whose arm is always maintained at a fixed base potential. In all of the examples shown herein, this potential is that of the cathode or filament, usually known as ground potential, and thus the switching device introduces practically no losses into the circuits. The entire coupling system is so designed that the single variable condenser C, having the proper capacity range, is equally useful for tuning, alternatively, over fthe low-frequency, orlong-wavelength, band and the high-frequency, or short-wavelength, band, and over still other bands, if necessary. Reference to the figure will disclose vthat in shifting from the low-frequency band to the high-frequency band, both primary windings L1 and L2 remain connected.

When used for the reception of low frequencies, the two primary coils L1 and L2, may be considered as a single primary winding associated with the secondaries L3 and L2. When functioning as a high-frequency coupling system, the e'ective primary winding may be considered as only L1, and the secondary as only L3, because the electrical constants are chosen so that the primary L2 has relatively small coupling with the secondary L2. This condition is brought about by a proper selection of the constants of the various coils, both individually and with respect to each other, in the following manner: The high-frequency primary L1 may comprise a relatively small inductance closely coupled to the high-frequency secondary L3. The low-frequency primary L2 connected in series with the high-frequency primary L2, comprises a larger inductance than L1, and is but loosely coupled to the high-frequency secondary L2.

When it is desired to receive signals within the low-frequency band, the short-circuiting switch S2 is opened; thereby causing the entire secondary winding to be coupled to the entire primary winding, in which event necessary transfer of energy from the primary to the secondary is secured in most part by the relatively close coupling between the low-frequency primary L2 and the low-frequency secondary L4. A slight increase in mutual inductance between the entire primary winding and the entire secondary winding is secured as a result of the coupling between the high-frequency primary Ll and the entire secondary, but this is relatively small as compared with the mutual inductance between the low-frequency primary and secondary.

When it is desired to receive signals within the high-frequency band, the short-circuiting switch S2 is closed, thereby short-circuiting the low-frequency secondary L2. The transfer of energy from primary to secondary is then effected through the combined couplings of L1 and L2 with the high-frequency secondard L2. Obviously, the major portion of the coupling for this band is between Ll and La. The physical position of winding L2 with respect to L2 is so adjusted that the electrical effect produced by the low-frequency primary L2 upon the high-frequency secondary L3 is relatively small when the low-frequency secondary L11 is short-circuited.

Much attention must be paid to the proper choice of the electrical constants of the various coils and to the physical, and consequential electrical, relationship of these coils. In some cases, for example, the two frequency bands here under discussion, it has been found desirable to design the primary coils so that their maximum inductances, when considered in conjunction with their distributed capacities and the capacities of the components (including the tuning condenser) and connecting wires connected to them, are resonant at frequencies higher `than the highest frequency of the respective tuning bands of the amplifier. If the resonant frequency of the primary circuit comes within a tuning-frequency band, the result may be y a materialreduction in the amplification and a corresponding' decrease in selectivity over a large part of that band.

The physical placement of certain of the coils produces other electrical effects which must also be considered in calculating the final erformance of the complete transformer. or example, short-circuiting the lowfrequency secondary coil L2 has a pronounced effect upon the high-frequency secondary L2, if these two coils are placed too close together. This effect varies with the dimensions and electrical characteristics of the respective windings, but in transformers such as herein described, the too close proximity of the short-circuited coil L,1 to coil L3 results in a pronounced increase in resistance in the coil La; thereby decreasing the amplification {nd the selectivity within the high-frequency and.

As has been noted, this ligure includes a neutralizing circuit Cn, N2, N1, connected in series between the grid electrode and the lilament system.` Since in this specific neutralization system the neutralizing coils N2 and N2 are electromagnetically coupled to plate coils L1 and L2, the neutralizing condenser Cn being connectedto the grid of the vacuum tube, the neutralization is effected in the plate circuit. The principle of plate-circuit neutralization is explained in Hazeltine U. S. Patent No. 1,533,858, especially as illustrated in Fig. 2 thereof, which system has been slightly .modified for application to the coupling system of the present invention. Accordingly, neutralizing coils N,L and N2, connected in series, are included in the coupling system of Fig. 2; N1 being closely coupled to high-frequency primary L1; and neutralizing winding i 2 being closely coupled to low-frequency primary L2. By properly proportioning the six different windings represented in the diagram, and by properly positioning them with respect to each other, it is thus possible to obtain entirely satisfactory neutralization over both frequency bands'.

The circuit arrangement of Figure 3, insofar as the coupling system between tubes A1 and A2 is concerned, is similar tol that of Figure 2. Here, however, an antenna system is coupled to the first radio-frequency amplifying tube A1 (as in a radio receiver) by a coupling system fundamentally based upon the intertube coupling system hereinbefore described. The antenna circuit is designed so as to have a capacitive reactance throughout lthe entire operating frequency range including both bands. The secondary inductances L10 and Ll1 are normally identical with secondary inductances L2 and L4. The primary inductances L8 and L9 may be identical with primary inductances L1 and L2 but this depends upon the antenna employed. The low-frequency band curve of Figure 4 is a fair representation of the curves of both bands of the'antenna coupling system of Figure 3. A short-circuiting switch S for the low-frequency primary L9 is included because the impedance of this coil may be too high in the high-frequency band to be effectively short circuited by `closing the secondary switch S. It should be noted that the switching arrangement provided in the circuit of Figure 3 may be made structurally simple and electrically efiicilentbecause all the switch arms are at ground potential.

The curves of Figure 4 illustrate specifically the approximate amplification/frequency characteristics of the antenna coupling system of Figure 3. Since the antenna circuit is resonant at a frequency between 300 and 500 kilocycles, the highest points on the two amplification curves correspond to these two operating frequencies nearest the resonant frequency of the antenna' circuit. The two amplification curves are lowest at 150 and 1500 kilocycles, the two operating frequencies farthest removed from the resonant fre.- quency of the antenna circuit, but the amplification is still sufficiently great at all frequencies Within the two operating bands.

In the circuit diagram of Figure 2 the input circuit of the first amplifying tube A1 is represented as an impedance Z, usually a tuned input circuit or an untuned antenna circuit; and the source of plate potential is shown as a battery B.

The system shown in Figure 2 may be used to couple an antenna to an amplifier, in which case the extreme terminals of Ll and L2 may be connected to the antenna and ground respectively, instead of to the. plate electrode and filament circuit of a vacuum tube, as illustrated. v

In no case in the drawings herewith are the filament heating circuits of the vacuum tubes shown completed for the reason that it is now well-known in the art that the filament or cathode of a vacuum tube may be heated by any of several methods. Likewise, the plate, or anode circuit of the second vacuum tube A2 in each gure is shown uncompleted; but it is to be understood that such plate circuits may be completed in the same manner as the plate circuit of the tube Al-and so on, fof as many amplifying stages as may be desired, the last stage being coupled to a detector or some other appropriate device.

Fig. 5 illustrates further modifications of Fig. 2 by which the low-frequency primary winding L2 is purposely loaded either by means of the distributed capacity of the winding itself, or by a physical condenser connected to the winding so that the whole may be resonant at a pre-selected frequency between the two tuning-frequency bands.

The neutralizing windings shown in other figures have been omitted from Figure 5 for the purpose of simplifying the diagram. It is to be noted that such windings have no bearing on the amplification characteristics of the arrangement of this ligure. (For the present discussion We lmay lassume that the dotted connection` above condenser C5 counects the plate battery, B, to the lower teryminal of inductanceLL., ,i.,e., vthat the addi-- Vtional resonlantcircuit C5, .L5I is not connected.) Thepurpose of this modification is to provide a nearly level amplification/fre- U quency curve over the high-frequency tuning band. This result is accomplished by arranging the separate coils of the entire transformer structure so that the low-frequency primary winding L2 has a definite' coupling with high-frequency secondary coil i L2. Since the coil L2 has more turns than the coil Ll, the former may have considerableelectromagnetic coupling with the secondary coil L3 oven though it is not as close to L3 as is coil L1. The inductance of L2 being frequency band and the minimum frequency of the high-frequency band, such as 450 kilocycles. Then the desired coupling conditions will be that the coupling between L1 and L3 predominates at the higher frequencies of the high-frequency band; and the coupling between L2 and L3 predominates at the lower frequencies of the high-frequency band. yIt is to be understood, of course, that the secondary coil L4 is presumed to be short-circuited when the above-defined conditions obtain.

The graphical representation of the manner in which these relations vary is given in Fig.' 5a which shows the amplification/frequency curves with only coil L1 in the primary circuit; with only L2 and C2; and with both coils functioning together in the improved manner. The upper curve, which is practically a horizontal line between 500 and 15.00 kilocycles (i. e., the high-frequency band) indicates that nearly constant over-all amplifi-A cation is attained.

The above description of the operation of the circuit of Fig. 5 is concerned only with the high-frequency band, because, as is well known, the undesirable characteristics ofamplifiers and coupling systems are usually more pronounced at the higher frequencies. If,

however, it is desired that the low-frequency band shall also have a substantially level overall amplification, the same principles may be applied. To illustrate; the primary coil L5 and its associated condenser C5 have been added to the figure, and for the present discussion may be considered to be together connected in the platecircuit of the amplifying tube A1, which is the primarycircuit of the coupling system. To accomplish this connection the dotted line above condenser Cls in the figure may be considered removed. For operation over the low-frequency band the switch S1 is opened. The inductance of coil L is made much greater than that of L2; and the capacity C, of such value that the circuit C5, L is resonant at a frequency lower than 150 kiloc cles, such as 100 kilocycles. The coil Ls is then so placed relative to coil L4 that the coupling between L5 and L4 predominates at the lower frequencies of the low-frequency band; whereas the coupling between L2 and L4 predominates at the higher frequencies of the low-frequency band. Although Fig. 5a is shown to represent only the high-frequency band, it can as well be considered to represent the amplification over the low-frequency band as just described, since the typical amplification/ frequency characteristics of the two bands will be practically identical if designed in accordance with the foregoing description. It is to beunderstood that if more than two frequency bands are included, additional circuits corresponding to C5, L5 may be connected in order that the additional frequency bands may likewise present uniform amplification characteristics.

A coupling arrangement such as shown in Fig. 5 is especially useful when employed to couple tubes of the-four-element or shielded-grip type. In such event, the circuit constantsrequirc certain modification depending upon the internal impedance of the tubes employed.

A successful physical embodiment of a coupling transformer structure designed in accordance with this invention is illustrated in Fig. 1 which is a cross-sectional view. Reference to the figure will disclose that the device includes an insulating tube 1 upon which are wound and supported the various windings already described. The tube 1 has an outside diameter of 1% inches and a len h of 31/2 inches. In this structure, the hig -frequency secondary winding L3 is wound in a single layer over the upper portion of the insulating tube, while the comparatively fewer turns of the high-frequency neutralizing winding N1 are placed in a single layer immediately on top of the low-potential end of the secondary winding; the high-frequency primary winding L1 being wound in a single layer immediately over the neutralizing winding, the layers being separated by insulation such as celluloid of about .O1 inch thicknes. These three windings thus comprise a complete high-frequency transformer. The low frequency transformer is wound upon and supported by the same insulating tube 1 on the lower portion thereoef. y It also comprises three windings, but here the neutralizing winding N2 is wound in a single layer on the insulating tube, the primary winding L2 being wound on and immediately over the neutralizing winding, the low-frequency secondary winding L4 of the multi-layer type being placed over the primary winding L2, an insulating layer being placed between the windings, as before. The spacing between the high-frequency transformer and the lowfrequency transformer, along the axis of the insulating tube 1, should be such that the two transformers have a negligible magnetic influence upon each other. The exact spacing required depends largely upon the frequency bands for which the different transformers are designed, and also upon the degree of amplification per stage and upon the over-all amplification of the complete amplifier. In general, this spacing must be about 1/1 inch. In the structure illustrated, the low-frequency primary and neutralizing coils L2, N2 are spaced 1/4 inch from the high-frequency secondary L3, and the low frequency secondary L4 is spaced fig inch from the high-frequency secondary L3.

Under some circumstances, such as when it is inconvenient to provide the required spacing due to other space limitations in the apparatus, or when the chosen frequency bands require it, it isnecessary to electroma etically shield or screen the separate trans ormers one from the other by means of a metallic shield, individual to each transformer, and electrically connect to the external shield or can 4 which is preferably placed so as completely to surround the entire structure, except in some instances at the bottom. As illustrated in Fig. 1, this can may be so shaped that it is spaced about the same distance from the windings of both transformers; i. e., the contour of the can may follow in general the average contour of the transformer windings, although the electrical effect does not change much if the diameter of the can is constant. The transformer structure may be secured tc the can with suitably Vpositioned machine screws or rivets 6, and spaced therefrom by spacers 3. If required, similar screws and spacers may likewise be provided at the upper end of the transformer, although these are not shown in the drawings. To connect the various windings of the transformer permanently in their respective circuits, terminal lugs 2 may be provided around the inside of the insulating tube 1; three of such terminals being shown in the gure. In order that the entire transformer structure, including the can, may be rigidly secured to a supporting panel, or the like, a flange or a series of feet, indicated in the drawings by reference character 5, is included. lf more than two frequency or wavelength bands are required, additional vwindings (see Fig. 5) similar to those already illustrated and describedmay be provided, in which event the insulating tube 11 would be slightly longer than shown.

The particular transformer structure illustrated in Fig. 1, and described herein, merev as a single typical embodiment of the pressent invention, was designed for use in a circuit of the type illustrated in Figs. 2 and and was eminently successful in a coupling system tunable over the two frequency bands already mentioned. The transformers had the followingv dimensions Iwhen employed with a vacuum tube having a grid-plate capacity of 4.5 auf and mutual conductance of 1000 micro-mhos:

Coupling transformers SeZf-inductance L1,L2 (with L4 short-circuited) 58 ph N1',N2 (with L4 short-circuited) 58 lah LUL2 (with L.1L not short-circuited) 7 0 ,ah N1,N2 with L4 not short-circuited) 70 uh L3 (with L.,I short-circuited) 233 uh Lal;14 (with L4 not Short-circuited) 3580 p.

M utuaZ inductance L1,L3 (with L4 short-circuited) 21.5 uh L1,L2 and vL3,L4 (with L4 not short circuited) 212 all Ooef/icient ofcoupling L1,L2 and N1',N2 90% (approximately) Dimensions All coils, except low-frequency secondary L4, wound with No. 38 (American gauge) enameled copper wire: L3,

Tuning condenser, 0 or 0') Minimum capacity 15uuf (approximately) M aximum capacity 400 ,raf (approximately) Neutralieing condenser capacity C.. 4.5 auf (approximately) Fig. la and Fig. 15 show the approximate amplification characteristics of the highfrequency transformer and the low-frequency transformer, respectively, of the structure illustrated in Fig. 1.

I claim:

1. A radio-frequency transformer structure including an insulating form upon which are wound a plurality of sets of windings, each set of windings comprising a complete transformer operating over a substantially different frequency band.

2. A radio-frequency transformer structure including an insulating form upon which are wound a plurality of sets of windings, each set of windings comprising a complete transformer operating over a substantially different frequency band, and a single metallic shield eii'ectively enclosing said structure.

3. A radio-frequency transformer structure including a supporting form upon which are mounted a plurality of sets of windings, each set of windings comprising a complete multiwinding transformer, the 'maximum radius of one ofsaid sets of windings being greater than that of another of said sets of windings,

and ametallic shield effectively. enclosingl said structure and radially spaced substantially the same distance fromeach set of j windings, whereby the electrical effect of said metallic shield is substantially the same upon all of said sets of windings. v

.4. A radio-frequency transformer structure including a supporting form upon which are mounted a plurality of sets of windings each set of windings comprising a complete multi-winding transformer operative over a substantiallydiferent frequency band, the

maximum radius of the set of windings operings and a set of low-frequency windings mounted upon a common insulating form, the set of high-frequency windings including a single-layer primary winding and a single-layer secondary winding wound one over the other, the set of low-frequency windings including a multi-layer secondary winding and a single-layer primary winding wound one over the other, said sets of windings being spaced apart longitudinally along said common form. j

6. A radio-frequency transformer structure comprising a set of high-frequency windings and a set of low-frequency windings supported upon a non-magnetic form, the set of high-frequency windings comprising a. secondary winding, a neutralizing winding, and a primary winding supported upon `said form in the Ordernamed, and a set of low-frequency windings comprising a neutralizing winding, a primary winding and a secondary winding supported upon said form in the order named.

7.v A radio-frequency transformer structure i comprising a set of high-frequency windings and a set of low-frequency` windings supported upon a non-magnetic form, the set of high-frequency windings comprising a secondary winding, a neutralizing winding and a primary winding supported upon said form inthe order named, and a. set of low frequency windings comprising a neutralizing winding, a primary winding and a secondary winding supported upon said form in the order named, said sets of windings being spaced apart longitudinally along said nonmagnetic form.

8. A radio-frequency transformer structure comprising a set of high-frequency windings and a set of low-frequency windings supported upon an air-core non-magnetic form, the set of high frequency wind-- ings comprising a single-layer secondary winding, a single-layer neutralizing winding, and a single-la er primary winding supported upon said rm in the order named, the set of low-frequency windings comprising a single-layer neutralizing winding, a single-layer primary winding, and a multilayer secondary winding supported upon said form in the order named.

9. A radio-frequency transformer structure comprising a set of high-frequency windings an'd'a set of low-frequency wind-1 ings supported upon an air-core non- 'magnetic form, the set of high-frequency windings comprising a single-layer secondary winding, a single-layer neutralizing winding and a single-layer primary winding supported upon said form in the order named, the set of low-frequency windingscomprising a single-layer neutralizing winding, a single-layer primary winding and a multilayer secondary winding supported upon said form in the order named, said sets of windings being spaced apart longitudinally along said common form.

l0. A radio-frequency transformer structure comprising a set of high-frequency windings and a set of low-frequency windings supported upon an air-core non-magnetic form, the set of high-frequency windings comprising a single-layer secondary winding, a single-layer neutralizing winding and a single-layer primary winding supported upon said form in the order named, the set of low-frequency windings comprising a single-layer neutralizing winding, a singlelayer primary winding and a multi-layer secondary winding supported upon said form in the order imed, said sets of windings being spaced apart longitudinally along said form, and a metallic shield secured to and effectively enclosing said structure.

11. A' radio-frequency transformer structure comprising an air-core form upon which are supported a set of closely-coupled'lowfrequency primary and secondary windings,

and a set of high-frequency windings includcy windings spaced apart longitudinally t along a non-magnetic supporting form, the two sets of windings being substantially without magnetic coupling therebetween.

In testimony whereof I aiiix my signature.

WILLIAM A. MACDONALD. 

